Which coding rule applies according to ICD-O-3 regarding tumors that overlap boundaries of categories?

The correct answer highlights the importance of using subcategory codes when the origin of a tumor cannot be determined. In cases where a tumor exhibits overlapping boundaries between two categories, proper coding is essential for accurate classification and research purposes. The use of subcategory code .8 signifies that the exact origin of the tumor is unspecified, which is a relevant approach when a definitive categorization cannot be established.

In contrast, using the appropriate 6th digit behavior to code morphology focuses on how to indicate the biological behavior of a tumor rather than addressing the issues arising from tumors that exhibit overlapping characteristics. The other choices may address related coding principles but do not specifically apply to the scenario of overlapping tumor boundaries as effectively as the selection of the subcategory code .8 does. Thus, the reference to subcategory .8 is particularly pertinent for maintaining clarity and precision in tumor classification in coding practices.